The invention relates to a method of and an apparatus for treating wood chips and in particular for improving the wood chips properties particularly in pulping processes of the pulp and paper industry. In said method the chips are divided into various fractions on the basis of a fragment size before any other treatment phases of the chips.
In pulping processes wood chips treating methods, in which over-thick (e.g. above 8 mm) chip fragments are separated from the wood chips by a screen and directed to be treated by a chip compressor, have been used for years. Known chip compressor structures are described, for example, in U.S. pat. Nos. 4,953,795 and 5,385,309, Finnish patent application (No.) 911 972 and Finnish utility model (No.) 2412. The chip compressor basically comprises two adjacent conveniently profiled rolls arranged to rotate in relation to parallel rotation axes. The chips to be treated are fed between the rolls.
Advantages gained by treating over-thick chips with compressors are thoroughly described, for example, in U.S. pat. No. 4,953,795. In brief, using compressor treatment the cooking properties of over-thick chips are improved to the level of accept-size chips.